There has heretofore been known a choke transformer for power circuit, including a flat magnetic core (hereinafter simply referred to as the core), a coil wound around the core, and two terminals formed at both ends on a surface of the core and electrically connected to both ends of the coil (see, for example, Japanese Laid-open Patent Publication No. 11-243021). In this choke coil, lead wires are connected to the terminals by high-temperature solder, and the terminals are bonded to the surface of the core with a conductive adhesive such that the lead wires are inserted between the terminals and the core.
Moreover, with the widespread use of the Internet, digital TV, and the like, a pulse transformer has been recently put into practical use as a transformer apparatus to efficiently transmit a pulse signal handled in a digital circuit. Particularly, in a LAN interface device mounted on information equipment such as a personal computer or audiovisual (AV) equipment combining audio and visual, a pulse transformer is used for the purpose of insulation and noise removal. The pulse transformer has the same configuration as that of a power circuit transformer intended for voltage conversion. Specifically, primary-side and secondary-side windings are wound around a core and are insulated from each other, and thus there is no electrical conduction therebetween. The pulse transformer is the same as the power circuit transformer in that signal transmission is performed by magnetic coupling and a voltage proportional to the number of windings is induced.
As illustrated in FIGS. 1A and 1B, a transformer apparatus 2 includes a transformer 10 placed therein. The transformer 10 is mounted in a case 20 having a structure in which gull-wing-shaped (L-shaped) external terminals 30 are drawn from two sides. The size of the case 20 having the transformer 10 mounted therein is about 10 mm in length, 18 mm in width, and 2 mm in height. The size of a core 12 in the transformer 10 mounted inside is about 2 to 4 mm in diameter.
When a winding 14 wound around the core 12 in the transformer 10 mounted inside the case 20 is connected to the external terminal 30, it is generally performed to wind an end (hereinafter referred to as a conductor) 60 of the winding 14 around a winding section 32 of the external terminal 30 and then connect the conductor 60 with solder or the like.
Note that FIG. 1B omits illustration of the solder to connect the conductor 60 of the winding 14 wound around the winding section 32 of the external terminal 30 to the winding section 32. When the conductor 60 of the winding 14 is wound around the winding section 32 of the external terminal 30, the conductor 60 of the winding 14 is wound around the winding section 32 in a tensioned state. Thus, tension is generated to remove slack in the conductor 60 of the winding 14.
In the transformer apparatus illustrated in FIG. 1B, when the transformer 2 is mounted on a circuit board while maintaining the tension generated in the conductor 60 of the winding 14 wound around the core 12, the conductor 60 of the winding 14 is thermally expanded by heat from the solder in a solder reflow process.
However, when the transformer apparatus 2 is cooled after the mounting thereof on the circuit board, the expanded conductor 60 of the winding 14 is thermally contracted, leading to a risk of disconnection of the conductor 60.
It is an object of one aspect of the present disclosure to provide a transformer apparatus and a method for manufacturing a transformer apparatus, which may reduce disconnection of a transformer winding while maintaining high reliability between the transformer winding and an external terminal.